Nuclear power plant uses heat generated by the nuclear fission of uranium. The nuclear fuel material currently in widest use is uranium oxide pellets. In a typical production process of the uranium oxide pellets, a lubricant is added to and mixed with a starting material of uranium oxide powder, and then pre-molded under a predetermined pressure, e.g., about 1 ton/cm2 to produce a slug. The slug is pulverized to obtain granules. Subsequently the lubricant is added to and mixed with the granules obtained and then compression-molded to form a compact, i.e., green pellets having a theoretical density (TD) of about 50%. The compact is sintered in a hydrogen-containing gas atmosphere to produce uranium oxide pellets. The uranium oxide pellets obtained as described above have a TD of about 95.5% and a grain size of 6 to 10 μm. Crystal grains of the nuclear fuel pellets are of an equiaxed polyhedron. Recently, nuclear fuel for high burn-up and long fuel cycle have been developed in order to enhance the economic operation of nuclear power plant and minimize the amount of spent nuclear fuel. A sintered nuclear fuel pellet having a large grain size can improve the integrity of a nuclear fuel rod under high burn-up conditions by preventing the external release of fission products having a gaseous phase or corrosiveness from the sintered fuel pellets. Also, deformation characteristics at high temperatures are improved when grain size increases. As a result, the safety of the nuclear fuel rod can be improved by effectively decreasing stress induced on a cladding by the sintered fuel pellets during an operation. For this reason, research has been conducted into manufacturing sintered uranium-based oxide pellets having a large grain size as sintered pellets used in a nuclear fuel rod for high burn-up or ultra high burn-up. Since grain growth is achieved by means of the transfer of materials through grain boundaries, it is important to increase a transfer rate of materials through grain boundaries during sintering in order to manufacture a sintered pellet having a large grain size. Methods of increasing a sintering temperature or using additive elements have been disclosed in order to increase a grain size during the manufacturing of a sintered nuclear fuel pellet. Methods of dissolving additive elements and forming a liquid phase having a fast diffusion rate at grain boundaries are disclosed for the using of the additive elements.
The method of dissolving additive elements uses a phenomenon in which defects are formed when additive elements are dissolved in a uranium-based oxide and the transfer of materials is facilitated, such that a grain growth rate is increased. A method of sintering at low temperatures by dissolving surplus oxygen in UO2 is disclosed in U.S. Pat. No. 6,878,313 B2. In this patent, a process for decreasing a sintering temperature by increasing an oxygen partial pressure of a sintering gas to dissolve oxygen ions in UO2 lattices to form uranium (U) cation vacancies, and increasing a material transfer rate through the formed U cation vacancies is suggested. In addition, aluminum (Al), chromium (Cr), titanium (Ti), niobium (Nb), magnesium (Mg), vanadium (V), phosphorous (P), or silicon (Si) are known as additive elements. The additive elements are usually added in a range of a few ppm to a few tens of thousands of ppm, based on a weight ratio with respect to uranium cations in a sintered pellet, and the amounts of additive elements may differ according to the type of additive element. In the method of increasing a grain growth rate by forming defects through the dissolution of additives, an amount of additives has to be increased in order to obtain a defect concentration above a certain level. Also, defects in UO2 lattices formed by dissolution have a limitation of contributing to increase release rates of fission gases generated during irradiation in a reactor. That is, although a grain size is increased to suppress fission gas release, a suppressing effect on the fission gas release is offset due to an increase in a diffusion rate of fission product in the UO2 lattices. According to the results of studies by Killeen et al. [Journal of Nuclear Materials, 88 (1980), p. 177-184] and Kashibe et al. [Journal of Nuclear Materials, 254 (1998), p. 234-242], it is reported that Cr ions were dissolved in a UO2 pellet to exhibit a grain growth effect, but a suppressing effect on fission gas release was low due to an increase in a diffusion rate of a fission gas caused by defect formation in UO2 lattices. To overcome the foregoing limitation, methods for removing surplus oxygen by heat treating in a reducing atmosphere at a temperature lower than a sintering temperature or minimizing lattice defects by precipitating dissolved metal cations in a metal form are disclosed in U.S. Pat. Nos. 6,878,313 B2 and 6,221,286 B1, respectively.
Technologies related to the methods of increasing a grain size by allowing additive elements to form a liquid phase at grain boundaries near a sintering temperature are reported. In U.S. Pat. No. 4,869,866, a technology for manufacturing sintered UO2 having an average grain size of 37 μm by sintering at 1640° C. for 7 hours after adding 0.5 wt % of an alumino-silicate additive is disclosed. According to this patent, it is reported that the alumino-silicate additive forms a liquid phase at grain boundaries near a sintering temperature and grain growth occurs by considerably accelerating material transfer through the liquid phase. Bourgeois et al. [Journal of Nuclear Materials, 297 (2001), p. 313-326] report that when an oxygen partial pressure of a sintering gas is controlled to a specific value during the manufacturing of Cr-added sintered UO2, a Cr-compound additive forms a liquid phase during sintering to greatly increase the grain size of the sintered UO2. U.S. Pat. No. 6,221,286 B1 suggests a process, in which a Cr2O3-added UO2 green pellet is sintered in an oxygen partial pressure interval where a liquid phase is formed, and then, dissolved Cr is precipitated into Cr metal particles by annealing at low temperatures and low oxygen partial pressures. In the case of a process using a liquid phase, a grain size is determined by an amount of a liquid phase formed at grain boundaries, and since a portion of additives is dissolved before reaching a liquid phase formation temperature or a portion of the liquid phase is dissolved in grain interiors during liquid phase sintering, a large amount of additives may be necessary to obtain grains of a desired size. For example, a detailed method of manufacturing a Cr-added sintered UO2 having a large grain size disclosed in U.S. Pat. No. 6,221,286 B1 is as follows. A sintered pellet is manufactured by sintering a Cr2O3-added uranium oxide green pellet at 1700° C. for 4 hours in a wet hydrogen gas atmosphere having a moisture/hydrogen gas ratio of 1.7%, and then, a sintered nuclear fuel pellet with precipitated Cr is manufactured by annealing the sintered pellet at 1300° C. for 5 hours in a dry hydrogen gas atmosphere having a moisture/hydrogen gas ratio of 0.05% or less. In the foregoing method, the added Cr2O3 maintains a Cr2O3 phase while the temperature of the green pellet is increased to near 1680° C., and dissolution occurs in a portion of the added Cr2O3. A portion of the Cr2O3, remaining without dissolution, contributes to grain growth by forming a liquid phase at 1680° C. or more. Thereafter, dissolved Cr is precipitated into Cr metal particles in a low temperature annealing process. Since a large amount of initially added Cr2O3 is dissolved before the forming of a liquid phase and only a portion contributes to grain growth, a large additive amount more than 1000 ppm is necessary.
When a grain size of a uranium-based oxide is increased by using additives, it is necessary to minimize an amount of the additives for obtaining the same grain size if possible. The reason is that the additive elements increase diffusion rates of fission products by dissolving UO2 lattice as well as reducing neutron economic by lowering an amount of a U charge or absorbing neutrons. Therefore, developments of new technologies capable of significantly increasing a grain size as well as minimizing an amount of additives are necessary.
A method, which improves a grain growth effect by maximizing an amount of a liquid phase existing at a sintering temperature through maximally suppressing dissolution of additives while the temperature of a green pellet is increased to the sintering temperature, is disclosed in Korean Patent No. 10-0964953. This patent is characterized in that an added Cr-compound is reduced to Cr at 1500° C. or less and a Cr phase is maintained. Thereafter, a process of sintering at 1650-1780° C. in a gas atmosphere having an oxygen potential of forming a Cr liquid phase is included. A sintered pellet manufactured by the foregoing process may have a larger grain size because an amount of a liquid phase formed during sintering based on the same addition amount is greater in comparison to a sintered pellet manufactured by the process suggested in U.S. Pat. No. 6,221,286 B1. However, the process suggested in Korean Patent No. 10-0964953 has a limitation in that the additive liquid phase formed is rapidly dissolved into UO2 lattices because oxygen partial pressure rapidly increases at a high sintering temperature. According to the results of a study by A. Leenaers et al. [Journal of Nuclear Materials, 317 (2003), p. 62-68], it is reported that the solubility of Cr ions in UO2 lattices rapidly increases when temperature and oxygen partial pressure increase at 1550° C. or more. Therefore, the process suggested in Korean Patent No. 10-0964953 has limitations in that duration time of liquid phases formed at grain boundaries is too short to completely contribute to grain growth, and more than a certain amount of additives is necessary.